1. Field of the Invention
This invention concerns oxygen enriched air/fuel combustion steam generation apparatus and methods, more especially in terms of reduced heat transfer surface areas as the percentage of oxygen in the combustion air is increased to 100 percent. In this way a new design of steam generation apparatus is promoted, characterized both by lower investment costs and operating costs.
2. Related Art
The steam generation research and development community faces an important challenge in the years to come: to produce increased amounts of energy under the more and more stringent constraints of increased efficiency and reduced pollution. In order to fulfil both of these requirements, oxygen-enriched air/fuel combustion appears like an attractive candidate, since it has already proven to lead to significant process improvements in other industrial applications, such as fuel savings, production increase or reduced emissions.
U.S. Pat. Nos. 6,282,901 and 6,314,896 disclose methods of oxygen enrichment in existing air/fuel combustion steam generation apparatus, involving a certain ratio between the oxygen enrichment and the flue gas recirculation, such that the heat transfer patterns are maintained relatively unchanged. The purpose of the present invention is quite different, since the present invention aims at creating a new design of steam generation apparatus, specially adapted for oxygen-enriched air/fuel combustion, preferably with oxygen enrichment higher than 90%. As used herein the term “boiler” will be used to denote generic steam generation apparatus, which includes boilers producing steam for power generation through turbines, as well as steam generation for other uses. U.S. Pat. No. 5,265,424 discloses and advanced furnace boiler system using oxygen as the oxidant. However, the patent fails to teach how to calculate heat transfer areas for the various heat transfer surfaces, and thus does not even mention or recognize the reduction in heat transfer surface areas possible using oxygen-enriched air, or industrially pure oxygen.
Oxygen-enriched combustion (OEC) has become a popular technique employed in a series of industrial applications, such as glass, steel, aluminum and cement manufacturing, to name only a few. The employment of the oxygen-enriched technique has proven to lead to significant process improvements in these industries, such as fuel savings, production increase, waste processing, and the like. Presently, there are applications where the employment of oxygen enriched-combustion has not yet started to be applied on a large scale. One of these applications is boilers, where very large amounts of fuel are used for combustion purposes.
Existing steam generation apparatus have widely ranging steam outputs, requiring an energy input from a few hundred kW to thousands of MW. However, the very large investment required for a new boiler, together with the already high thermodynamic efficiency of existing boilers make the introduction of operational changes relatively difficult to implement. The boiler operators are reluctant to introduce modifications in the boiler characteristics, due to possible changes in water vapor properties (temperature, humidity, and the like). Different heat transfer patterns in the various areas of the boiler (combustion space, convective regions) will lead to different local vaporization/superheating rates of the steam, with direct impact on the boiler tubes. Local vapor superheating may lead to lower heat transfer coefficients, therefore to local boiler tube overheating, eventually causing cracks in boiler tubes. It is therefore crucial, when retrofitting an existing air/fuel combustion boiler to combust oxygen-enriched air with fuel, to maintain relatively unchanged the heat transfer patterns as originally designed, in order to produce safely the designed vapor throughput.
Basically, the use of oxygen-enriched combustion has two consequences to the boiler: it reduces the mass fraction of nitrogen, and it increases the adiabatic temperature of the flame. It is thus clear that the oxygen-enriched combustion can dramatically affect the heat transfer patterns in a system characterized by both radiative and convective heat transfers. While the increased flame temperature has a beneficial role on the radiative heat transfer, the diminished flow rates and temperature levels in the convective part of the installation may lead to lower heat transfer rates in this region. This means that for systems where the radiative heat transfer is the main heat transfer mechanism, such as cement kilns or glass furnaces, the oxygen enrichment can be used as such, without further modifications. However, for systems where convective heat transfer is important, changes to the installations have to be performed, in order to maintain the design parameters of the system unchanged, without modifying the heat exchanger structure.
Several inventions have already dealt with oxygen enrichment in steam generating boiler operation, promoting different methods to retrofit existing installations. The oxygen boosting can actually be used in connection with:                Increase of production (steam throughput), with the same boiler design;        Redesign of the convective part, to ensure the same production;        Fuel staging, allowing gas temperature and mass flow rate increase, in order to maintain the same convective heat transfer as in the initial design.        Flue Gas Recirculation, to maintain general heat transfer patterns essentially the same as the air-based combustion.        
None of these solutions have been readily accepted in the art, for one or more reasons. Therefore, there exists a need in the art for a new boiler design to apply oxygen-enriched combustion in steam generators. There is especially needed designs which allow taking advantage of the usual benefits of oxygen-enriched air/fuel combustion, while avoiding the above risks and constraints linked to the retrofit of existing boilers.